Hardening gives Eijiro the power to harden any part of his body.
Hardening protects Eijiro from most physical threats. Once activated, Eijiro becomes capable of enduring what would be otherwise lethal hazards, like tons of falling metal and multiple explosions, without leaving a scratch. Even bullets are unable to pierce his skin.
Aside from increased defense, Hardening also grants Eijiro offensive power by turning his hardened body rough and sharp like rock, increasing his physical strength and allowing him to cut enemies by grazing them. This all leads to Hardening being a highly practical Quirk for Hero work, which is frequently combat-oriented.
According to Tetsutetsu Tetsutetsu, the hardness granted by Hardening can be improved upon when Eijiro, in his hardened state, takes more damage overtime. This fact, prior to Tetsutetsu stating it, was displayed as part of Eijiro's training, when he, in his hardened form, was continuously attacked by Mashirao Ojiro's Tail, and with each hit, the strength and durability of Eijiro's Quirk increased. The reason behind this may be that Hardening adapts to the delivered damage, and after being deactivated, the Quirk resets itself before reactivated again, in which the density of Hardening is more powerful compared to it's previous use.
One of the drawbacks to this Quirk is that there is a limit to how much damage the hardened skin can withstand, causing Hardening to slowly dissipate, and Eijiro loses stamina from struggling to keep his Quirk active.  But, when he encountered Kendo Rappa, Eijiro showcased the ability to refortify his hardened form at will.
Furthermore, Hardening is noted to lack long-range potential, limiting Eijiro's combat repertoire and forcing him to take battles up close. Hardening can also only protect Eijiro from physical damage but not thermal damage such as very high or low temperatures. During the Paranormal Liberation War Arc, Eijiro was able to endure Dabi's blue flames, which are significantly stronger than regular flames. Tetsutetsu then elaborated that, while Eijiro doesn't get burned by the flames, he may still feel the high temperatures.
Hardening first manifested on Eijiro when he was just three years old, as he was rubbing his right eye after waking up. Eijiro ended up cutting himself, leaving a scar around his right eyebrow. Originally, this Quirk only slightly hardened Eijiro's skin, but he was able to improve its effects ever since through intense training. Before that, he could barely defend himself against rocks.
In combat, Eijiro is very straightforward and reckless, letting his hardened body take the opposing hits as he counterattacks with hard punches and other close-range moves. As even bullets cannot pierce his skin when his Quirk is active, Eijiro is capable of turning himself into a human shield for allies or defenseless civilians. During rescue operations, he has to be extra careful with victims as his rugged body can hurt them.
In desperate situations, Eijiro can increase his own hardness to its maximum level, becoming almost invincible temporarily. In another desperate situation, his battle with Rappa demonstrates that even if his hardening were to break down he can follow it up with another hardening which increases his durability and duration in battle. His tactics remain the same, however, due to the simple nature of his Quirk.
- Like several other Quirks, Hardening has its own signature sound effect: "SKLIT".
- This Quirk is very similar to Steel, and the two appear to be roughly equal in terms of power and effectiveness.
- The effects of Hardening and Steel are very similar, but the latter Quirk turns the user's entire body into hardened steel instead of just hardening their skin. The two appear to be roughly equal in terms of power and effectiveness, although Tetsutetsu notes that his body can only become so hard due to transforming into metal, whereas Eijiro can become increasingly tougher.
- This Quirk, in terms of function, is the inverse of Stiffening, which consists of hardening foreign objects instead of one's own body.
- My Hero Academia Manga: Chapter 13 (p. 8).
- My Hero Academia Manga: Chapter 25.
- My Hero Academia Manga: Chapter 41.
- My Hero Academia Manga: Chapter 280.
- My Hero Academia Manga: Chapter 133 (p. 5).
- My Hero Academia Manga: Chapter 134 (p. 4).
- My Hero Academia Manga: Chapter 214.
- My Hero Academia Manga: Chapter 217.
- Hardness - Wikipedia article about the property Eijiro's Quirk is based after.